When looking at the electron configurations of ##Na## and ##Ne## we can notice that the first ionization energy of sodium is less than that of Neon for the following reasons:

Distance from nucleus: The electron removed from sodium is being removed from a higher energy level ##n=3## than that of Neon ##n=2##.

Effective nuclear charge : Even though sodium's nuclear charge is ##+11## and that of Neon is ##+10##, however, due to the distance from the nucleus, the nuclear charge effect is very minimal in this case.

Shielding effect: The electron removed from sodium is shielded from the attraction of the nucleus by ##10" electrons"## however, the electron removed from Neon is shielded by ##2" electrons"## only.

Electron-electron repulsion: The electron repulsion effect is usually considered when the electrons removed are at the same distance from the nucleus and they are subjected to a similar nuclear charge and shielding effect. In this case, the electron-electron repulsion effect is minimal.